This invention is related generally to plasma torches which are generally used for cutting, welding and spray bonding and to an overall improved design for such torches.
Plasma torches, also known as electric arc torches, are commonly used for cutting, welding and spray bonding of workpieces and operate by directing a plasma consisting of ionized gas particles toward the workpiece. In the operation of a typical plasma torch, such as illustrated in U.S. Pat. Nos. 4,324,971, 4,170,727 and 3,813,510, assigned to the same assignee as the present invention. A gas to be ionized is supplied to the front end of the torch in front of a negatively-charged electrode. The welding tip which is adjacent to the end of the electrode at the front end of the torch has a sufficiently high voltage applied thereto to cause a spark to jump between the electrode and welding tip thereby heating the gas and causing it to ionize. A pilot DC voltage between the electrode and the welding tip maintains an arc known as the pilot or non-transferred arc. The ionized gas in the gap appears as a flame and extends externally from the tip. As the torch head or front end is brought down towards the workpiece, the arc jumps from the elctrode to the workpiece since the impednace of the workpiece current path is lower than the impedance of the welding tip current path.
In conventional torches, the negatively-charged electrode is typically made of copper with a tungsten insert and current flows between the tungsten insert and the torch tip or workpiece with the torch is operated. Tungsten is oxidized easily at high temperatures so that if the gas to be ionized is air, the tungsten insert becomes oxidized and is consumed rapidly. Therefore, the gas to be used for creating the plasma is typically an inert gas, such as nitrogen or argon.
Frequently, a secondary gas flow is also provided in conventional plasma torches for various different purposes. The most common purpose of a secondary gas flow immediatly adjacent and surrounding the electric arc is to cool the torch and the workpiece. In a cutting operation, cooling the workpiece will result in a straighter kerf and therefore a cleaner cut. In a welding operation, cooling the workpiece will result in a less deformed or more accurate weld caused by the flow of molten metal. In conventional plasma torches, two gas lines are provided: one for supplying the plasma forming gas and the other supplying gas for the secondary gas flow. If different gases are used for the plasma forming gas and the secondary gas, operation of the torch will require two gas supplies. Having to use gas lines is inconvenient to torch operators and using two gas supplies is expensive. Therefore, it is desirable to provide a plasma torch which requires only one gas line and only one gas supply.